Cryoablation, or killing tissue with low temperatures, is often used to treat cardiac arrhythmia conditions such as atrial fibrillation. However, when treating particular regions of tissue, through thermal energy interaction or the like for example, it may be difficult to direct or control the depth and intensity of the heat transfer. The delivery of thermal energy or other therapeutic modality, such as radiofrequency or cryogenic applications, may not necessarily be contained to the exact region or depth desired for treatment, as the tissue may have varying therapy-conducive properties affected by the surrounding physiological environment. While thermal control or precision may be of more concern with certain treatment modalities, such as radiofrequency, microwave, and/or cryogenic treatment procedures, it is often desirable to limit thermal treatment or exposure to just the tissue desired. Failure to do so may otherwise negatively and adversely affect surrounding tissue structures or organs that are sensitive and susceptible to undesired damage.
For example, when attempting to treat cardiac tissue, there are many nearby sensitive tissue structures that may react adversely to thermal applications. In particular, when thermally treating or ablating tissue in or about the heart, it is essential that critical physiological structures such as the phrenic nerve, sinoatrial node, and the like are not inadvertently destroyed through such ablation therapy. The phrenic nerve is made up mostly of motor nerve fibers that produce contractions of the diaphragm and thus affect breathing and respiration patterns and conditions. In addition, the phrenic nerve provides sensory innervation for many components of the mediastinum and pleura, as well as the upper abdomen, especially the liver, and the gall bladder.
The phrenic nerve is generally referred to in two segments: the right and left phrenic nerves. Both phrenic nerves run from C3, C4 and C5 vertebrae along the anterior scalene muscle deep to the carotid sheath. The right phrenic nerve passes over the brachlocephalic artery, posterior to the subclavian vein, and then crosses the root of the right lung anteriorly and then leaves the thorax by passing through the vena cava hiatus opening in the diaphragm at the level of T8. The right phrenic nerve passes over the right atrium, proximate the superior vena cava (SVC). The left phrenic nerve passes over the pericardium of the left ventricle and pierces the diaphragm separately.
The right atrium and left atrium of the heart may be the location or origin of arrhythmias or other physiological maladies and thus targeted for tissue ablation in order to remove or otherwise remedy the abnormal electrophysiological occurrence. In thermally treating or ablating select cardiac regions, the phrenic nerve may be at risk of being similarly, although unintentionally, ablated. This could severely impact the normal respiratory functioning of the patient. Such injury can manifest as a transient phrenic functional block, transient phrenic nerve palsy (PNP), or longer-term phrenic nerve injury. These injuries reduce respiratory function and can require many weeks or months to resolve. In the worst cases, this reduced function requires mechanical ventilation assistance to maintain respiration. As such, the risk of such unintentional and undesirable destruction or application of thermal energy to this and other cursory structures compels a desire to monitor or otherwise detect potentially-damaging consequences during treatment.
Such monitoring is typically performed using pacing the phrenic nerve and using continuous fluoroscopy during the ablation to visualize a consistent diaphragmatic response, or palpation of the abdomen to confirm diaphragmatic movement. Both methods require vigilance on the part of the operator, and can distract the physician from the main focus of the diagnostic or treatment procedure at hand. Further, in the case of fluoroscopic monitoring, the patient is exposed to increased x-ray radiation.
Phrenic nerve palsy, which may cause hemidiaphragm paralysis, is the most encountered complication of cryoablation for atrial fibrillation. Various techniques have been developed to monitor the phrenic nerve, but all revolve around delivering pacing energy to the phrenic nerve from within the SVC, at a convenient location proximate a portion of the phrenic nerve, such as the left phrenic nerve. Typically, a focal catheter is used to stimulate, or pace, the phrenic nerve, but the catheter often shifts during the procedure, resulting in loss of capture. To the physician, this may appear as though the diaphragm has stopped contracting, and the physician may decide to prematurely stop the ablation procedure.
Known mapping catheters, such as the ACHIEVE® mapping catheter (Medtronic, Inc., Minneapolis, Minn.), may have a distal portion that is expandable to bring one or more electrodes in contact with target tissue, such as an inner circumference of a pulmonary vein. Such a device may be suited for phrenic nerve monitoring, but the expanded distal portion may have an outer diameter that is too small to bring the electrodes in contact with an inner diameter of the SVC, which may be larger than that of a pulmonary vein. Further, currently known mapping catheters may lack the stiffness or strength to be used without, for example, a cryoballoon device.
Accordingly, it is desirable to provide a device and system for monitoring the phrenic nerve from within the SVC that is more stable than currently known mapping catheters, and that includes an expandable distal portion that is adjustable to ensure contact between electrodes and target tissue in any of a variety of vessel sizes.